There are two questions.
The first is is there is any death of heart tissue prior to the operation and if so, how much. Or if there has been structural deterioration ofn the heart (enlarged heart, for example). This requires a consultation with your physician.
The second is the potential damage to the healing process.
As mentioned, three months is about the minimum.
There seems to be strong evidence that daily vitamin C supplements significantly improves the strength of the tissue when healing.
I have not had a bypass, but have broken ribs several times and they have taken a long time to heal. Ten to eleven months.
Just to add, the leaflet I took home after surgery stated, nothing heavier than a kettle for the first 6 weeks, then slightly heavier objects such as dragging/pushing a vacuum cleaner around was fine. After three months no limitation as long as the healing process hasn't been hindered in any way.
I would have said everyone is different, but the average would be three months. It takes on average 6 weeks for the average persons bones to heal, but there is muscle etc to consider.